Mutation occurs continuously and can be spontaneous. It can also happen because of:
Ionising radiation includes gamma rays, X-rays and ultraviolet rays. The greater the dose of radiation a cell gets, the greater the chance of a mutation.
Mutations could cause different genes to be switched on or off, and this could create a different or faulty protein to be synthesised. For example, if the protein is an important enzyme, the specific substrate might not fit into the substrate binding site. If it is a structural protein such as collagen, it might lose its strength.
However, most DNA mutations do not alter a protein, they only alter it slightly so its appearance or function is not changed.
There are many different types of mutations which can arise in DNA.
Original gene structure
These mutations may change the activity of a protein, in a coding part of the DNA, or it might change how the genes are expressed if the change is in a non-coding section of DNA. These might result in phenotype changes or they might appear hidden, and be unnoticed. Alternatively, they might result in a serious consequence, such as genetic disease such as cystic fibrosis.